The development of a constitutive theory for materials with variable bonding (materials which respond to applied deformations by rupture of particle-to-particle bonds) is described. The theory is applied to materials in which bond breakage can be represented as a rate process. The result is a constitutive law which represents the stress response to an applied strain as the sum of a linear viscoelastic stress component and a stress component due to 'bonding' (bond breakage). The /bonding stress component is linearly related to the bonding. rate; the bonding state is governed by a nonlinear ordinary differential equation. The constitutive law was applied to data from tests on TP-H1011 propellant. (The tests utilized cyclic loading of the propellant to provide a mixture of damaged and undamaged propellant responses.